[XLA] Add new optimization that sinks constants into while loop bodies

Example transformation:

   state = (..., const, ...)
   while (pred(state)) {
     (..., v, ...) = state
     use(v)
     state = (..., v, ...)
   }

 =>

   state = (..., const, ...)
   while (pred(state)) {
     (..., v, ...) = state
     use(const)
     state = (..., v, ...)
   }

PiperOrigin-RevId: 195143323